The present invention generally relates to sub-micron resonant tunneling diodes and a process for their fabrication, and more specifically to sub-micron resonant tunneling diodes with improved peak-to-valley current ratios.
As logic circuits continue to decrease in size and increase in speed, currently utilized semiconductor devices such as transistors may prove limiting. As such, novel types of semiconductor devices are under development to meet the requirements of high-speed circuitry. One such device is the resonant tunneling diode (RTD). Very generally, in devices of this type, the semiconductor layer structure includes thin layers of quantum wells that can permit low resistance, high speed electron tunneling. Such devices potentially provide increased high-speed switching, increased device density, and reduced power dissipation in logic circuits.
Currently, research on RTDs focuses on utilizing new fabrication technologies to reduce the size of these devices to the sub-micron level. Decreasing the size of these devices may reduce the capacitance and correspondingly increasing the maximum frequency of oscillation for the device and, further, may reduce the requisite peak current for the device.
Decreasing the size of an RTD, however, has been limited by the increase in leakage current with the increase in surface-to-area ratio. Increased leakage current becomes a dominant factor in the current-voltage dependence of the device, and the increased leakage results in an increased valley current relative to the peak current. Thus, the peak-to-valley current ratio is reduced.
Nomoto, et al. reported successful fabrication of RTDs down to 20 nm in diameter. However, as noted above, the peak-to-valley ratios for these devices were limited by their dimensions. The peak-to-valley current ratio for the 20 nm device was less than 1.1 and thus the device is not practically useful in a circuit application. The 80 nm diameter device fabricated by the same researchers exhibited a somewhat better peak-to-valley current ratio of 1.2, with an attractively low peak current of 50 nA, but also exhibited a limiting peak current density of 103 A/cm2.
Smith, et al. reported fabricating a sub-micron RTD with an improved peak current density of 1.4xc3x97105 A/cm2 and a peak-to-valley current ratio of 2.0. However, while this device was 100 nm across, it was also 10 mm long, and thus had a net area of 1 mm2. This larger device size, while facilitating an improved peak-to-valley current ratio, also leads to higher power loss and higher peak current.
Therefore it is desirable to have a sub-micron resonant tunneling diode with a favorable peak-to-valley ratio, lower peak current requirements and lower capacitance than larger RTDs.
The present invention relates generally to sub-micron diodes and more particularly to a sub-micron resonant tunneling diode with an improved peak to valley ratio. The device comprises a substrate in contact with a bottom contact layer and an ohmic metal contact in contact with the bottom contact layer. A top cap and quantum wells layer is sandwiched between a top metal contact and the bottom contact layer. The top of the ohmic metal contact and the top of the top metal contact are substantially planer. A passivating substance in contact with the substrate fills all voids from the substrate to the top of the ohmic metal and top metal contacts, substantially enveloping the bottom contact layer, the top cap and quantum wells layer, the ohmic metal contact, and the top metal contact. The top of the ohmic metal and the top metal contacts are clear of the passivating substance.
In one embodiment of the present invention the layers of the device comprise the following:
Substrate:
InP
Bottom contact layer:
approximately 100 angstroms of undoped InGaAs;
and approximately 3000 angstroms InGaAs doped
at approximately 1xc3x971019 cm3 
Top cap and quantum wells layer: approximately 375 Angstroms thick InGaAs doped at
approximately 1xc3x971019 cm3 
approximately 70 angstroms of undoped InGaAs
approximately 5.1 monolayers of undoped AlAs
approximately 17 angstroms of undoped InGaAs
approximately 12 angstroms of undoped InAs
approximately 17 angstroms of undoped InGaAs
approximately 5.3 monolayers of undoped AlAs
approximately 70 angstroms of undoped InGaAs
A novel method for fabricating the sub-micron resonant tunneling diode is disclosed, which comprises providing a layered material comprising: a top cap and quantum wells layer, a bottom contact layer, and a substrate. The top cap and quantum wells layer is patterned to allow selective etching to the bottom contact layer. The top cap and quantum wells layer as removed by etching as patterned. An ohmic metal is deposited to contact only the bottom contact layer and a top metal contact is selectively deposited, using a pattern, on the top cap and quantum wells layer. This is followed by removing the top cap and quantum wells layer except where masked by the top metal contact and removing the non-local bottom contact layer to form a device. The device is then passivated using a passivating substance in contact with the substrate that fills all voids from the substrate to the top of the ohmic metal and the top metal contacts, substantially enveloping the following the bottom contact layer, the top cap and quantum wells layer, the ohmic metal contact, and the top metal contact. The passivating substance covering the top of the ohmic metal contact and the top metal contact is etched away revealing the contacts for testing and operation. In this invention the ohmic metal and the top contact metal are substantially planer. In a preferred embodiment the ohmic metal and the contact metal further comprise a cap layer of Titanium.